COLOUR, FORM, AND MOVEMENT. 



65 



An ordinary snow flake consists of many of these crystals 

 in a loose entangled mass. They are delicate and are 



Fig. 8. Examples of snow crystals. 



frequently broken in their fall through the air; yet 

 although they melt quickly as we examine them closely, it 

 is not difficult to get a view of perfect examples. 



2. Quartz. Quartz crystals con- 

 sist of six-sided prisms, topped 

 by six-sided pyramids. They are 

 known as rock crystal ; perfect 

 examples are found where they 

 have grown in cavities. The 

 number and arrangement of the 

 axes is the same as in the snow 

 crystal (Fig. 9). Quartz occurs in 

 granite and sandstone, but per- 

 fect crystals cannot be seen in 

 these ; it is the commonest mineral 



in the earth's crust. Fig ^ of Quarte 



o A^ AT j. IT showing crystals. 



3. Alum. Alum crystallises 

 in regular octahedra. There are 



three axes of the same length, intersecting each other 

 at a right angle. Exhibit a crystal or draw one. 



